1. Field of the Invention
The present invention relates to a process and system for the recovery of carbon dioxide from a carbon dioxide, co-solvent and contaminant stream emitted from one or more process tools which are operated in a discontinuous manner.
2. Description of Related Art
Carbon dioxide (CO2) based systems have become increasingly important in the electronics industry, particularly in the fabrication of semiconductor components. CO2 based systems may be utilized in numerous operations including chemical fluid deposition, photoresist deposition and photoresist development and removal. For example, supercitical CO2 may be utilized to remove photoresist (i.e., contaminant) from semiconductor wafers.
A wafer having a contaminant on the surface thereof is placed in one of several cleaning chambers that includes a process tool. Carbon dioxide and one or more co-solvents, such as water or acetone, are injected into this chamber and the wafer is cleaned. During at least part of this cleaning process, the chamber temperature and pressure meet or exceed the supercritical temperature and pressure of carbon dioxide. Carbon dioxide, co-solvents and contaminant are then exhausted from the chamber such that the chamber pressure is reduced to ambient. To minimize the amount of carbon dioxide and co-solvent lost when the process tool depressurizes, the valve that allows the carbon dioxide, co-solvent and contaminant-containing stream to leave the process tool is located as close to the process tool as possible. The clean wafer is then removed from the chamber.
Typically carbon dioxide, co-solvent and contaminant-containing streams which are exhausted from the process tool need to be scrubbed of any co-solvent and contaminant before they are vented to the atmosphere because these substances may be hazardous. Further, it is sometimes desirable to purify and recycle at least a portion of the carbon dioxide contained in the exhaust stream to minimize CO2 consumption and overall cost.
Several carbon dioxide recovery systems have been proposed in the related art. For example, U.S. Pat. Nos. 4,349,415 and 4,877,530 disclose processes in which the carbon dioxide application (i.e., the process tool) operates at a constant pressure that exceeds the triple point of carbon dioxide. In these disclosures, carbon dioxide is employed in a continuous extractor to remove an extract from a raffinate, forming extract- and raffinate-enriched streams. The extract-enriched stream contains most of the carbon dioxide and passes continuously to purification and recycle means. The raffinate-enriched stream also contains a small amount of carbon dioxide and is sent to a phase separator, which produces further enriched raffinate-containing liquid and carbon dioxide-enriched vapor streams. The carbon dioxide-enriched vapor stream is sent to a holding tank to dampen any fluctuations in flow and is then compressed and recycled.
Since the processes described in U.S. Pat. Nos. 4,349,415 and 4,877,530 operate continuously, the extractor pressure is never reduced to ambient levels. Therefore, the phase separator may operate at any pressure that would accommodate downstream processes. It is desirable to operate the;phase separator at a pressure that exceeds ambient pressure so that the carbon dioxide-enriched vapor stream may be transferred to other equipment, such as the holding tank, without first compressing this stream. Operating the phase separator at a pressure in excess of ambient pressure would also reduce the required holding tank volume and compression power.
If the process application (i.e. the extractor) described by U.S. Pat. Nos. 4,349,415 and 4,877,530 were operated in batch, as described in other related art, several problems would result. The process application pressure would likely need to be reduced to ambient level during its operation. This would necessitate operating the separator at ambient pressure. Therefore, compression would be required to transfer the carbon dioxide-enriched vapor stream to other equipment, such as the holding tank. Alternatively, the separator could be operated at a pressure in excess of ambient pressure. This would necessitate sending any vapor that remains in the extractor below the separator operating pressure to vent as a vent stream. This vent stream would exist as a multi-phase stream, likely as some combination of vapor, liquid and solid. Transfer of this multi-phase stream to vent would be very difficult and would have a deleterious effect on downstream equipment due to, for example, solid and liquid deposition. It is possible to compress the vent stream so that it may be sent to the separator. However, this would require the use of further compression equipment, resulting in significant cost increase. Further, since the carbon dioxide application disclosed in these patents is continuous, maintaining constant flow to the downstream processes is not a concern.
International Patent Document WO 02/085528 describes a process that employs a single separator vessel which operates at a wide range of pressures for upgrading and recycling liquid or supercritical carbon dioxide leaving a carbon dioxide application. The vessel, which is referred to as an expander-concentrator, operates in batch mode. Liquid leaving the carbon dioxide application is pumped and then sent to a high-pressure liquid holding tank. Liquid taken from this holding tank is routed to the expander-concentrator, which is physically located within the holding tank.
When the liquid level in the expander-concentrator reaches the desired value, its feed is discontinued. The pressure associated with the expander-concentrator is then gradually reduced, generating a vapor stream that initially contains high levels of co-solvent and contaminant but becomes co-solvent and contaminant-free as the expander-concentrator pressure is reduced. When the co-solvent and contaminant level associated with the vapor stream falls to acceptable levels, the vapor stream is released and recycled.
Since the expander-concentrator operates in batch mode, a continuous vapor stream is not generated, which may be deleterious to downstream equipment. Other deleterious effects of this phase-separation system include high cost due to use of specialized equipment and difficult design of the compressive/pumping device, especially if corrosive substances are present. Finally, it is difficult to transfer the carbon dioxide, co-solvent and contaminant-containing stream leaving the carbon dioxide application to the holding tank using a pump since this stream changes from liquid phase to multi-phase to vapor phase as the carbon dioxide application depressurizes. Moreover, as the expander-concentrator pressure approaches ambient pressure, vapor carbon dioxide leaving the phase-separator requires further compression means, which are resistant to the corrosive elements entrained therein.
Finally, the related art describes the use of multiple phase-separators to separate carbon dioxide from the carbon dioxide, co-solvent and contaminant-containing stream leaving a carbon dioxide application. U.S. patent application Ser. No. 2001/0050096 describes such a process. In the process described therein, the pressure associated with the carbon dioxide, co-solvent and contaminant-containing stream leaving a process tool is decreased. The resulting intermediate pressure stream is sent to a heated, intermediate-pressure phase separator, which generates a carbon dioxide-enriched vapor stream and a co-solvent and contaminant-enriched liquid stream. The carbon dioxide-enriched vapor stream is filtered and sent to a first condenser from which substances with a higher vapor pressure than carbon dioxide are vented. The co-solvent and contaminant-enriched liquid stream leaving the intermediate-pressure phase separator is depressurized to somewhat greater than ambient pressure and sent to a low-pressure separator, which generates a co-solvent-enriched vapor stream and a contaminant-enriched liquid stream. The co-solvent-enriched vapor stream is sent to a second condenser, where substances more volatile than co-solvent are vented. The contaminant-enriched liquid stream leaving the low-pressure phase separator is collected for disposal.
One of the disadvantages associated with the multiple phase-separators of U.S. patent application Ser. No. 2001/0050096 is that the intermediate-phase separator and most operations located downstream of it are designed to operate continuously. To ensure continuous operation, carbon dioxide bypasses the tool and is fed to the intermediate-pressure phase separator when carbon dioxide, co-solvent and contaminant-containing fluid is not discharged from the process tool. This represents a process inefficiency, since power expended to pressurize carbon dioxide that is normally fed to the process tool is discarded upon bypass.
Further, as the intermediate-pressure phase separator operates continuously, its pressure cannot be reduced to near ambient. Therefore, the process tool pressure cannot be reduced to ambient by discharging its contents into this separator. To reduce the process tool pressure to ambient, fluid existing at a pressure equal to and less than that associated with the intermediate-pressure phase separator is discharged to another vessel that is independent of all previously mentioned vessels. This vessel is heated to completely vaporize all substances that enter it. The resulting vapor is sent to a vent scrubbing system. It is likely that this vessel will need to be heated to greater than ambient temperature to completely vaporize its contents, since some co-solvents and contaminants will have a very low vapor pressure. As the vapor leaving this vessel is transferred to the vent scrubbing system, it will cool due to heat leak. As a result, these very low vapor pressure substances will re-condense and deposit on process piping, creating the multi-phase flow issues described above. These deleterious effects are exacerbated if the co-solvents are corrosive.
An additional disadvantage associated with this process is that the carbon dioxide-enriched vapor leaving the intermediate pressure phase-separator and ultimately recycled to the process tool is likely to have a significant co-solvent/contaminant level associated therewith, since the vapor pressure associated with these substances is not insignificant. Therefore, the process tool may be contaminated. Further, if corrosive co-solvents are employed, they may damage the compressors or pumps that pressurize the recycled carbon dioxide downstream of the first condenser.
U.S. patent application Ser. No. 2002/0023662 describes a process that uses multiple distillation columns to separate a solvent and contaminant-containing stream leaving an extraction application. As described therein, the extraction application is contaminant removal from solid adsorbents, such as clays. Supercritical carbon dioxide is listed as a potential solvent. Solvent and contaminant-containing liquid leaving the extractor is sent to a holding tank to eliminate fluctuations in flow caused by batch extractor operation. Liquid is pumped from this holding tank and sent to a first distillation column, which produces a first solvent-enriched stream and a first contaminant-enriched stream. The first solvent-enriched stream is recycled to the extractor without further purification. Therefore, a multi-staged distillation column is required. Distillation columns require a continuous feed, necessitating the holding tank. Further, pure liquid solvent must be fed to the top of the column because the column is reboiled. Since the solvent and contaminant-containing stream is not pure liquid solvent, some of the product solvent stream must be condensed and recycled to the column, resulting in poor efficiency.
The first contaminant-enriched stream is sent to a second separation distillation column that produces a second solvent enriched stream and an essentially pure contaminant stream. A distillation column is required, since the contaminant is a desired product. The solvent enriched stream is condensed, pumped and recycled to the first separation means. The essentially pure contaminant stream is collected for disposal or re-use.
A further disadvantage associated with this system is that the holding tank pressure can not be reduced to near ambient, since carbon dioxide does not exist as a liquid near ambient pressure. Therefore, the extractor pressure cannot be reduced to ambient by discharging its contents into the holding tank. To reduce the extractor pressure to ambient, fluid contained in the extractor at pressures equal to and less that that associated with the holding tank is discharged to the second distillation column. This presents significant operational concerns, since such columns separate poorly, if at all, with a discontinuous feed. Further, if the solvent is supercritical carbon dioxide, the second distillation column will not function well, since carbon dioxide can not exist in the liquid phase at ambient pressure. Carbon dioxide is likely to exist as a solid in the second column, which is likely to cause the column to plug.
If the first distillation column were replaced by a phase separator, a further disadvantage would be that the carbon dioxide-enriched vapor leaving this phase separator would contain high levels of contaminant since the vapor pressure associated with these substances is not insignificant. Since further separation means are not employed, the extractor would be contaminated. Further, if corrosive co-solvents are employed, they may damage compressors or pumps that pressurize the recycled carbon dioxide. Therefore, the use of phase separators is unacceptable.
To overcome the disadvantages of the related art, it is an object of the present invention to provide a process and system for the recovery of carbon dioxide from a discontinuous carbon dioxide stream emitted from at least one batch process tool.
Another object of the invention is to allow the process tool to vent to ambient pressure.
A further object of the invention is to transfer streams containing corrosive, toxic or hazardous substances such as acids or bases to further purification and/or vent-scrubbing systems.
Yet another object of the invention is to dampen process condition fluctuations associated with the feed to the further purification systems while the process tool is operating.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, the drawings and claims appended hereto.